The present inventions relates to material handling apparatus, and more particularly, to a lightweight, portable conveyor having parallel endless belts with upstanding pushers for lifting building materials onto rooftops.
Residential construction frequently involves the lifting of substantial amounts of heavy building materials, such as tiles, tarpaper, shingles and the like onto roofs. It is inefficient to do this by manual labor. The high cost of forklifts and other heavy lifting machinery, along with their requirement for skilled operators, makes such equipment undesirable for many residential construction projects. Also, accidents can occur when heavy lifting equipment is used to lift large loads onto rooftops.
U.S. Pat. No. 4,366,900 discloses a portable, lightweight conveyor capable of carrying substantial amounts of building materials onto rooftops in a short amount of time. It includes a telescopic frame which can be collapsed during transportation and pulled out to its full length for operation. A pair of endless belts are entrained about pulleys rotatably mounted at opposite ends of the frame so that the belts can travel about the pulleys in spaced apart parallel relationship. The pulleys at the rearward, lower end of the frame are driven by an electric motor through a gear reduction mechanism. Due to tolerance variations, the pulleys do not have identical diameters and the belts do not have identical lengths, although these dimensions are made as close to each other as is practical under the circumstances. Therefore, when the pulleys at the rearward end of the frame are driven at the same rotational speed, upstanding load retaining pushers or cleats on the belts tend to move away from transverse alignment. Obviously, if misalignment becomes too severe, loads cannot be properly retained by the pushers and will fall off of the conveyor.
To solve the foregoing problem, U.S. Pat. No. 4,366,900 includes a mechanism for synchronizing the movement of the two belts to maintain the load retaining pushers directly opposite each other. Specifically, magnetic position indicators on the belts are sensed by a control circuit which selectively actuates clutch couplings to briefly stop either drive pulley so that the other belt can catch up. While this conveyor is a servicable apparatus and represents an improvement over previous conveyors, the clutch couplings are complex, expensive and subject to failure as a result of excessive wear. Also the momentary stopping of a drive pulley tends to be abrupt and can dislodge loads from the belts.
Accordingly, it would be desirable to provide a portable, lightweight conveyor of the twin belt type having an improved belt synchronizing mechanism. If the belts were to be permanently connected by longitudinally spaced links, the belts would still tend to travel at uneven speeds, resulting in unwanted drag on the pulleys on one side of the conveyor and a skewing of the transverse links. The use of such links would also dictate that no support beams, control mechanisms, rails, etc. could extend between the belts.
Substitution of a single wide belt for the twin belts is also not a desirable solution. Such belts tend to be heavy and expensive. Generally the control and driving mechanisms must be mounted within the confines of a single wide belt.
Driving each of the separate rear pulleys at different rotational speeds is also not desirable. First of all, to be economical, such a portable conveyor should use a standard AC induction motor. The RPM of such a motor can only be varied through expensive and complex electronic circuitry. Any mechanism for independently varying the speeds of the two drive pulleys to synchronize the belts would be undesirably complex.
The use of twin metal drive chains and sprockets in place of belts and pulleys is also not desirable. Such chains are heavy and expensive. They cannot readily be removed and folded when the telescopic frame of the conveyor is collapsed. Twin rubber timing belts with pulleys and teeth are also not desirable. Such rubber timing belts are also expensive. Furthermore, such belts have a tendency to stretch an unacceptable amount when the telescopic frame is extended and the belts tensioned with the forward crank and screw mechanism.